There are a number of systems which optically read a code pattern such as a bar code on a package or other object. For high speed applications, scanners operate effectively over only a narrow range of distance. The narrow operating distance arises from the need to reduce the diameter of an optical scanning beam to approximately the width of the smallest element to be read. For a bar code of black bars and white spaces, for example, individual bars can have a width as small as five mils (0.005 inch) or less. Many lasers commonly used in optical scanning systems emit a beam having a diameter of thirty to forty mils which must be reduced to eight mils or less to read the narrow bars of these smaller codes. Further, as the distance between the scanner and the code increases, faster clock elements are required because the beam travels a greater distance--and therefore at a faster rate--during each sweep as the range increases.
There are a number of situations in which it is not possible to maintain a uniform distance between the scanner and the code to be read. In the furniture industry, for example, a number of dramatically different sized boxes can pass beneath a scanner can the same conveyor belt. A large box containing a desk or chair can be forty or fifty inches high or more, whereas a box containing a table top is less than two inches in height.
Presently, there are several types of systems which attempt to accommodate a variable range between the code and the scanner. One system utilizes two or more sets of fixed focus scanners, one set focused at a close range of one to thirty inches, and a second set focused at thirty to sixty inches, for example. However, the scanners are expensive and complicate installation, operation and maintenance of the system. Further, due to divergence of the focal point of each set, resolution may not be acceptable at ranges intermediate to the close and far ranges.
Another system involves a vertically oriented scanner with two-position focusing. The system includes two additional optical devices for providing light beams across a conveyor belt, a lower team to detect the presence of a package and a higher beam which, when broken, commands the scanner to shift from far focus to near focus. All of the above systems only provide coarse focusing which dramatically limits their resolution of codes on variable sized boxes.